ANATOMY/MICROANATOMY/HISTOLOGY
Adipose tissue "fat" is formed by aggregations of fat cells (adipocytes) containing stored fat (lipid) in the form of single droplets of triacylglycerol (triglyceride). Fat tissue is comprised of clusters of adipocytes ranging from small fat cells (SFC) to large mature fat cells (MFC). They are typically spherical but may assume polyhedral shapes because of mutual deformation. A single fat cell is 95% fat by volume. The cell nucleus is displaced to one side by the accumulated lipid and the cytoplasm is reduced to a thin rim. Each individual fat cell has large numbers of hormone and other receptors in the cell wall.
There are many different types of receptors, including, alpha and beta adrenergic receptors, and insulin receptors. Other types of receptor include: sex hormone receptors, glucocorticosteroid receptors, and other classes of receptor. A single adipocyte (fat cell) may have as many as 5,000 to 10,000 receptors. The various receptors are made of proteins with complex three-dimensional conformations. Each receptor is specialized to recognize single, or occasionally several types of chemical or hormonal signals.
Each fat cell is surrounded by delicate reticular fibers. In the angular spaces between the cells are capillaries and nerve supplies that form a loose plexus throughout the tissue. Adipose tissue is subdivided into small lobules by connective tissue septa. This compartmentalization, visible with the naked eye, is most obvious in regions where the fat is subjected to pressure and has a cushioning or shock-absorbing effect. In other regions the connective tissue septa are thinner and the lobular organization of the tissue is less apparent.
Adipose tissue is distributed in the subcutaneous tissue but exhibits regional differences influenced by genes, age, sex, activity levels and eating habits. Adipose tissue collectively constitutes a large diffuse "organ" that is metabolically very active; it is primarily engaged in the uptake, synthesis, storage (accretion), and mobilization (release/depletion) of neutral lipid or fat. As a result of release, the caloric content of the lipid stored in the fat can be made available as heat or as energy to cells in other parts of the body. At body temperature, the lipid in adipocytes is present as liquid oil. It consists of triacylglycerol, each made up of three molecules of fatty acid esterified to glycerol.
Infants and young children have a continuous subcutaneous layer of fat. The panniculous adiposus is generally a uniform thickness over the entire body. As the young child grows to an adult, the fat layer thins out in some regions of the body but persists and grows thicker in certain sites of predilection. These sites differ between sexes and are largely responsible for the characteristic contour differences in male and female body form.
In the male, the principal areas of predilection are the neck and the region overlying the seventh cervical vertebra, the subcutaneous area overlying the deltoid and triceps, the lumbosacral region, and the buttocks.
In the female, subcutaneous fat is most abundant in the anterior neck, the breasts, the buttocks, the epitrochanteric region, and the anterior aspect of the thigh. Few blood vessels pass through subcutaneous fat into the overlying skin, which receives its nutrients through a subdermal plexus of blood vessels which run above the fatty layer.
In addition to the superficial subcutaneous fat deposits of varying thickness covering the body, there are extensive "deep" internal abdominal (visceral) accumulations in both sexes in the omentum, mesenteries, and retroperitoneal areas. All of these areas, to varying degrees, readily give up, or "release" by the process of lipolysis, their stored lipid during fasting.
Obesity is defined as an increase in the mass of adipose tissue. Humans accumulate excess body form, shape, or "contour-determining" subcutaneous adipose deposits in the body when energy intake derived from food exceeds daily energy needs. This may be accomplished by an increase in fat cell size or fat cell number or an increase in both fat cell size and number. Once obesity is established, weight reduction is accomplished by a decrease in fat cell size or volume and the number of fat cells remains relatively unchanged.
The diversity of human shape or contour seen in differing individual bodily fat accumulations depends on a complex combination of factors such as, age, sex, lifestyle, genetic and hormonal influences.